Phase changeable memory elements are well known nonvolatile memory elements. A unit cell of a phase changeable memory element includes a data storage element that is formed of a phase changeable material. The phase changeable material has two stable states. That is, the phase changeable material is either in an amorphous state or in a crystalline state in accordance with the temperature of supplied heat and/or the supply time of the heat. The phase changeable material in the amorphous state generally has a higher resistance than that of the phase changeable material in the crystalline state. Therefore, the amount of current that flows through the phase changeable material generally varies with the state of the phase changeable material. It is possible to determine whether the information stored in the phase changeable memory cell is logic “1” or logic “0” using change in the amount of the current. A compound of Ge, Sb, and Te, GST(Ge—Sb—Te), may be used as a well-known phase changeable material. Other phase changeable materials may be used.
Joule heat is used as the heat supplied to the phase changeable material. In particular, current may be supplied to a heating electrode connected to the phase changeable material to generate Joule heat that is to be supplied to the phase changeable material. It is possible to control the temperature of the heat supplied to the phase changeable material by controlling the amount of the supplied current.
A high temperature generally may be needed in order to change the state of the phase changeable material. For example, heat around the melting point of GST of about 630° C. may need to be supplied in order to make the GST amorphous. Therefore, there may be significant increase in the amount of current supplied for a programming or erasing operation. As a result, the power consumption of a phase changeable memory element may be high. Also, the channel width of the MOS transistor in the phase changeable memory element may increase in order to control a large amount of current, so that it may be difficult to highly integrate the phase changeable memory element.
FIGS. 1 and 2 illustrate forming a conventional phase changeable memory element.
Referring to FIG. 1, a lower insulating layer 2 is formed on a semiconductor substrate 1 and a heating electrode 3 is formed on the lower insulating layer 2. An upper insulating layer 4 that covers the heating electrode 3 is formed and the upper insulating layer 4 is patterned to form a contact hole 5 that exposes a predetermined region of the heating electrode 3. A phase changeable material film 6 is formed on the surface of the semiconductor substrate 1 and in the contact hole 5. The phase changeable material film 6 is formed of GST.
As shown in FIG. 2, the phase changeable material film 6 is patterned to form a phase changeable material pattern 6a connected to the heating electrode 3.
According to the above-described conventional phase changeable memory element and the method of forming the same, in order to reduce the amount of current for performing a programming or erasing operation, the width of the contact hole 5 may be reduced. That is, the contact area between the phase changeable material pattern 6a and the heating electrode 3 may be reduced to reduce the amount of current.
However, the aspect ratio of the contact hole 5 generally increases with reduction in the width of the contact hole 5 such that a void 7 may be generated in the contact hole 5. Moreover, since the GST is commonly formed by a sputtering method, the void 7 may be more easily generated. When the phase changeable material pattern 6a is made amorphous (for example, during a programming operation), heat of about 630° C. may be generated at the interface between the phase changeable material pattern 6a and the heating electrode 3. Therefore, Te, whose melting point is about 445° C., may first evaporate from the phase changeable material pattern 6a into the void 7. As a result, the components of the phase changeable material pattern 6a may change (that is, Te is lost). Therefore, the phase changeable material pattern 6a may lose its own characteristics such that the characteristics of the phase changeable memory element may deteriorate.
On the other hand, in order to prevent the void 7 from being generated, after depositing the phase changeable material film 6, the phase changeable material film 6 may be reflowed. A process of reflowing the phase changeable material film 6 may be performed at the temperature around or higher than the melting point of the phase changeable material film 6. A furnace thermal process and/or a rapid thermal process is commonly used as the process of reflowing the phase changeable material film 6.
The rapid thermal process and/or the furnace thermal process may be performed at a temperature no less than about 630° C. for dozens of seconds to several hours. As a result, the void 7 can be filled. However, since the process of reflowing the phase changeable material film 6 is performed for a long time (for dozens of seconds to several hours), Te of the phase changeable material film 6 may evaporate into the void 7 before the void 7 is filled. Therefore, the components of the phase changeable material film 6 in the contact hole 5 may change even after the rapid thermal process and/or the furnace thermal process is performed to reflow the phase changeable material film 6. That is, one part of the phase changeable material film 6 in the contact hole 5 may be heavily concentrated with Te and another part may be lightly concentrated with Te. Also, during the process of reflowing the phase changeable material film 6 in the rapid thermal process and/or the furnace thermal process, Te may evaporate from the top surface of the phase changeable material film 6. Therefore, the components of the phase changeable material film 6 around the top surface may change and the thickness of the phase changeable material film 6 may be reduced. As a result, the characteristics of the phase changeable material pattern 6a may deteriorate such that the characteristics of the phase changeable memory element may deteriorate.